This application is in response to NOT-OD-09-058, NIH Announces the Availability of Recovery Act Funds for Competitive Revision Applications. The mechanisms controlling skeletal development, growth and function continue to attract much research effort, but are far from clear. In the current Parent Grant, we studied the roles of Wnt/b-catenin signaling in growth plate function, and our novel hypothesis at the time was that 'Activation of Wnt/b-catenin signaling is required for chondrocyte hypertrophy and function and for progression and completion of endochondral ossification.'The data we obtained since and reported in several publications fully affirmed the validity of our hypothesis and did uncover the central importance of Wnt/b-catenin signaling in chondrocyte hypertrophy, matrix remodeling and apoptosis and in the seamless transition from cartilage to bone during endochondral ossification. Our studies and those by others have contributed significantly to the current public recognition of the importance of Wnt/b-catenin signaling in skeletal development. As our Parent Grant was progressing, we obtained related, intriguing and rather novel data on possible connections between Wnt/b-catenin signaling and endoplasmic reticulum (ER) stress. ER stress and unfolded protein response (UPR) are extremely popular in many fields of research because of their physiologic and pathologic roles, but have received relatively little attention in skeletal biology. Our preliminary data using transgenic mice now reveal that Wnt/b-catenin signaling stimulates ER stress/UPR and apoptosis in growth plates in vivo. Using primary chondrocyte cultures, we have found that acute activation of Wnt/b-catenin signaling enhances expression of CHOP and Bip (that are key UPR response genes) and leads to apoptosis under ER stress conditions. Our central hypothesis for this Revision Application is that that Wnt/b-catenin signaling interacts with ER stress/UPR signaling to regulate and topographically restrict apoptosis in growth plate chondrocytes. Our aims are: (1) to determine how CHOP and Bip expression is regulated by activation of Wnt/b-catenin signaling;and (2) to examine CHOP and Bip roles in ER-stress-induced apoptosis during Wnt/b-catenin signaling activation. This Revision Application grant will allow us to begin to fill glaring gaps in current information and will add novel goals to our Parent Grant. It will uncover regulatory interconnections between Wnt/b-catenin signaling and ER stress/UPR in growth plate function and establish a previously unsuspected paradigm in endochondral ossification. These regulatory cross-talk and circuitry are likely to be essential for growth plate homeostasis and function and may be deranged, and cause, a number of congenital and acquired skeletal diseases. PUBLIC HEALTH RELEVANCE: Project Narrative Cartilage plays essential roles in formation and growth of the skeleton. Dysfunction in cartilage due to congenital or acquired conditions can lead to defects in body structures, growth retardation and degenerative cartilage diseases. This project will provide novel information on mechanisms regulating behavior and function of cartilage cells and will pave the way to uncover the causes of skeletal diseases.